An acoustic sensor is a device to convert voice to an electrical signal and types of micro electro mechanical system (MEMS) acoustic sensors include a capacitive type and a piezoelectric type. A capacitive MEMS acoustic sensor is based on a principle of a condenser that enables two electrodes to face each other. Here, one electrode is fixed on a substrate and the other electrode is afloat in the air whereby the capacitive MEMS acoustic sensor moves in reaction to external sound pressure using a diaphragm. When the external sound pressure is received, the diaphragm vibrates and a capacitance value varies according to a change in a gap between two electrodes. Accordingly, current flows. The capacitive type has advantages in that it is stable and a frequency characteristic is excellent. Therefore, most conventional acoustic sensors use the capacitive scheme.
FIG. 1 is a perspective view illustrating a structure of a capacitive MEMS microphone having a flexible spring according to the related art, and FIG. 2 is a perspective view illustrating a structure of a capacitive MEMS microphone having a spring and a stop bump according to the related art.
Referring to FIG. 1, in the case of the conventional capacitive MEMS microphone, rigidity is intended to be decreased by inserting a flexible spring 15 in a portion in which a membrane 14 is connected to a substrate 11, in order to decrease rigidity of the membrane 14 and increase sensitivity of a microphone. However, in this case, deformation occurs in the flexible spring 15 due to a process stress and a package stress. Accordingly, the membrane 14 also becomes to move whereby a sensing gap 13 may vary or the membrane 14 may abnormally operate.
In FIG. 2, to decrease an effect of such deformation, proposed is a method of maintaining the sensing gap 13 between the membrane 14 and a backplate 16 to be constant while operating by inserting a structure 17 for maintaining an interval into the substrate 11 or the membrane 14. In this case, even though it is possible to maintain the sensing gap 13 between the membrane 14 and the backplate 16 to be constant while operating, it is difficult to decrease rigidity of the membrane 14 and not possible to decrease the effect from a package stress.
Accordingly, there is a need for a method that may achieve high sensitivity by decreasing rigidity of a membrane in a capacitive MEMS microphone and by decreasing the effect from a process stress and a package stress.